Deep Diffusion Doping of Macroporous Silicon

Arrays of ordered macropores in silicon have attractive applications in electronics, [1,2] photonic crystals, [3,4] micromachining, [5][6][7] and biosensors. [8,9] Such arrays consist of straight, vertical channels along the {100} crystallographic directions with surface openings ordered in a periodic lattice. The channels can have circular to square cross sections with typical diameters of 0.5-20 lm and a lattice period of about twice the diameter. Their aspect ratio (depth/diameter) can reach several hundreds. The macropores are prepared by anodic etching of weakly doped, n-type (100) Si in a HF:H 2 O electrolyte under back-side illumination. [10,11] A key step in the preparation of ordered arrays is pore seeding. It determines the pattern of microchannels by creating pits on the Si surface, at which the electrochemical pore formation is initiated. The pits are typically made by oxidation of the Si surface, followed by standard photolithography to open a lattice of windows in the SiO 2 layer. Subsequently, the Si is etched anisotropically in KOH through this mask to form inverted pyramids in the windows. Thus, a lattice of sharp pits is formed on the Si surface. During the following electrochemical anodization under back-side illumination, the etching current, carried by holes, concentrates at the pits, hence resulting in propagation of macropores from the pits. [10,11] This method gives excellent results if the typical lattice dimensions are larger than a few micrometers. On the other hand, patterning using standard contact-mask photolithography becomes difficult if smaller pores or lattice periods are needed. In this case, more sophisticated techniques, such as electron-beam or deep-UV lithography, have to be employed. A relatively simple alternative is laserinterference lithography (LIL), [12,13] in which two or three beams from a standard UV laser are employed to form an interference pattern (stripes or dots) on a photoresist-coated substrate. The interference pattern has an excellent periodicity over a large area with a pitch that can be controlled down to one half of the laser wavelength. However, using ordinary LIL one encounters problems when transforming the interference pattern into a surface topology by etching, due to image distortions caused by secondary interference effects within the photoresist. [13] Better resolution can be obtained by using additional antireflection coatings, [13] which adds to the complexity of this method. On the other hand, it has been shown that by using an interference dot pattern with a pitch of only 500 nm produced by a high-power pulsed laser one can directly form a periodic array of tiny pits on Si surfaces without using any photoresist or other coating. [14] However, the pits obtained are not deep enough to serve as efficient seeds for electrochemical macropore formation. Here, we present a new approach to seeding ordered pores by holographic laser patterning using a thin metal coating to transfer the interference image to a well-defined surface topology. The experim...

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confidence: 99%